The present invention relates to a displacement measuring device used for a hand-tool such as a digital vernier caliper, in particular, to a so-called capacitance-type measuring device for absolute measurement of positions based on a relative position of a movable element against a fixed element of a displacement sensor.
A growing number of small measuring devices with capacitance-type displacement sensors such as digital vernier calipers, digital micrometers, and height gauges are being used. A capacitance-type displacement sensor comprises a fixed element such as a main scale, a movable element such as a slider, the movable element being able to move against the fixed element. A large number of electrodes are disposed on the fixed element and the movable element. As the movable element moves against the fixed element, the capacitances between these electrodes change. The sensor outputs periodical changes of the capacitances as electric signals. Thus, the displacement of the moving element against the fixed element can be measured.
There are two types of displacement sensors which are categorized as incremental type and absolute type depending on the forms of output signals. An incremental type sensor continuously detects the cyclic signals so as to measure the displacement of a slider against a reference position. In contrast, an absolute type sensor measures an absolute displacement (position) of a movable element against a fixed element as opposed to the incremental type sensor. In the absolute type sensor, for example, depending on the shapes of electrode patterns formed on the fixed element and the movable element, cyclic signals with coarse pitches, medium pitches, and fine pitches are output. The phase information of these cyclic signals with these pitches is composed. Thus, an absolute displacement of the movable element is detected.
The theory of such a capacitance-type absolute displacement sensor is disclosed in the specification of U.S. Pat. No. 4,420,754. When such a displacement sensor is employed for a small measuring device, a signal processing circuit portion should be formed as an IC chip. In addition, a battery should be built in the device. To reduce the size of the battery, improve its service life, simplify its replacement procedure, and save natural resources, the average current consumption of the system should be reduced as much as possible.